add: send to a list of mailboxes

[lzjwlt]

Send to a list of mailboxes config like this: "notifications":{ "mail":{ "smtpHost":"xxx.com", "port":25, "username":"[email protected]", "password":"xxxxxxxx", "from":"[email protected]", "to":["[email protected]", "[email protected]", "[email protected]"] } },

[BobCashStory]

it woul cause breaking change for all using mail, could you do a support for old and new method ?